The present invention relates to a radiation detector used for a positron emission computed assisted tomography (PET), a single photon emission computed assisted tomography (SPECT) and the like, wherein the device detects a radioactive ray, i.e. gamma ray, emitted from a radioactive isotope (RI) administered to a subject and accumulated at a target region of the subject to thereby obtain a tomogram of an RI distribution at the target region. The present invention also relates to a method of producing the radiation detector.
A radiation detector is formed of a scintillator for receiving gamma rays emitted from a subject to emit light, and a photoelectron multiplier for converting the light emitted from the scintillator into an electrical pulse signal. A conventional radiation detector has a structure wherein a single scintillator corresponds to a single photoelectron multiplier. In recent years, a modified radiation detector has been developed to achieve high resolution with a minimum number of components. In such a modified radiation detector, a plurality of scintillator crystals is bundled into an array-form, and the photoelectron multipliers in the less number than that of the scintillator arrays are connected thereto, so that an incident position of the gamma ray is determined from an output ratio of the respective photoelectron multipliers.
In the recent method as described above, only the scintillator crystal, which the gamma ray enters, among the scintillator crystals constituting the array emits light. The incident position of the gamma ray is determined from an output ratio of the respective photoelectron multipliers receiving light emitted from the scintillator at a specific position. It is necessary to correctly determine the incident position for improving an image of a medical diagnosing device to which the radiation detector is applied. When the incident position of the gamma ray is determined from the output ratio of the photoelectron multipliers as described above, it is necessary to obtain a proper detection distribution in the adjacent photoelectron multipliers so that the distribution changes at a constant rate according to the light incident position. It is critical to properly distribute the scintillator light to the respective photoelectron multipliers in order to determine the incident position with high accuracy. To this end, various methods have been proposed.
Japanese Patent Publication (Kokai) No. 62-500957 has disclosed a radiation detector shown in FIG. 8. The radiation detector has a light guide disposed between scintillators and photoelectron multipliers. A plurality of barriers, i.e. light reflex barriers, is inserted into the light guide at various depths to determine the position. The radiation detector includes a plurality of scintillators 110 divided by slits 111 formed of a light reflex material or a light blocking member; a light guide 120 optically connected to the scintillators 110 and divided into small compartments with different depths by light reflex members or light blocking members; and four photoelectron multipliers 1301, 1302, 1303 and 1304.
In the radiation detector, the respective barriers 121 in the light guide 120 have lengths increasing toward an outer side from an inner side, so that the incident position of the gamma ray can be discriminated.
In the conventional radiation detector as described above, the light guide 120 is formed of an optically transparent material, and slits having predetermined depths are cut into the light guide 120 with a dicing saw or a wire saw. The light reflex members or light blocking members are inserted into the slits to form the barriers 121.
The conventional radiation detector as described above has the following problems. In recent years, a high-resolution radiation detector using super-sensitivity scintillators has been developed, wherein a large number of the scintillators are used as compared with the conventional radiation detector. Accordingly, a section of each scintillator becomes smaller than that of the conventional scintillator. In such a radiation detector, it is necessary to form the light guide optically connected to the scintillators with high accuracy, and to make a width between the barriers short so that the light transmission efficiency is not deteriorated.
In the conventional radiation detector, the slits with predetermined depths are formed in a block of an optically transparent material with a dicing saw or a wire saw, and the barriers are inserted into the slits. Accordingly, it is difficult to machine the block with high accuracy, and the slits tend to have rough surfaces and large widths. Also, in a case that the block is cut into nine pieces with the dicing saw or wire saw, and these pieces are assembled to form the slits, the process becomes complicated, resulting in high cost. Further, after the slits are machined or formed, the light reflex members are inserted. Accordingly, a gap is created between the light reflex member and the slit, thereby reducing the reflex efficiency. When the light output of the incident gamma ray is decreased, it is difficult to correctly determine the position, thereby reducing the whole image quality.
In view of the problems described above, the present invention has been made, and an object of the invention is to provide a radiation detector in which the position can be correctly determined and the whole image quality can be improved.
Another object of the invention is to provide a method of producing the radiation detector.
Further objects and advantages of the invention will be apparent from the following description of the invention.